Method of burning fuels



Jan. 11, 1966 P. FRASER ETAL 3,228,451

METHOD OF BURNING FUELS Filed Aug. 25, 1963 5 Sheets-Sheet 1 FIG I 242220 l8 l6 l4 l2 IO 8 6 4 AIR TO FUEL RATIO (lbs/lbs.)

Reg/bald Percy Fraser Maxwell See/y Waosnam INVE NTORS BY WWW ate lfi w ATTORN EYS Jan. 11, 1966 P. FRASER ETAL METHOD OF BURNING FUELS 5 Sheets-Sheet 2 Filed Aug. 23, 1963 1966 R. P. FRASER E METHOD OF BURNING FUELS 5 Sheets-Sheet 3 Filed Aug 23, 1963 Whm m wvEA/roi 121A! PM Af m- Jan. 11, 1966 R. P. FRASER ETAL METHOD OF BURNING FUELS 5 Sheets-Sham Filed Aug. 23, 1965 Jam 1966 R. P. FRASER ETAL METHOD OF BURNING FUELS 5 Sheets-Sheet 5 Filed Aug. 23, 1963 mv 03m 0% United States Patent i 3,228,451 METHQD OF BURNING FUELS Reginald Percy Fraser, Kingston Hill, Surrey, and Maxwell S. Woosnam, London, England, assignors to Urquharts (1926) Limited, Greenford, Middlesex, England, a British company Filed Aug. 23, 1963, Ser. No. 304,121 Claims priority, application Great Britain, June 25, 1957, 20,052/62 7 Claims. (61. 158-1175) This application is a continuation in part of our application Serial No. 743,913, filed June 23, 1958, now abandoned.

The present invention relates to a method of burning fuels, particularly those fuels known as heavy residual fuel oils.

For many reasons it is desirable to burn heavy residual fuel oil rather than refined fuel oils or other types of fuel in heat exchange apparatus and the like. However, in order to avoid producing air pollution products in the form of unburned carbon particles which appear at the exhaust end of the combustion apparatus as smoke, it has been found necessary to burn the fuel with an excess of air, that is to say air in addition to that just necessary for the complete combustion of the fuel. It has been found that when this is done, a relatively large amount of S0 is produced due to the oxidation of the S0 produced by the burning of the sulphur in the residual fuels.

Heretofore attempts have been made to overcome this problem of S0 production, which leads to corrosion of the combustion apparatus due to the formation of sulphuric acid. Ammonia injection at the base of the stack of the apparatus has been tried, and fuel additives are still being tried.

It has also been observed that S0 formation is avoided when the oxygen supply is reduced to a point where the proportion of oxygen to fuel is below stoichiometric proportions, and several theories have been proposed to account for this. However, when the oxygen is reduced, heavy smoke is also produced, which, in the light of recent efforts in the field of air pollution, is not only undersirable but also unpermissible in most installations.

It is an object of this invention to provide a method of burning such residual fuels at high intensity, that is to say with a production of more than half a million British Thermal Units per cubic foot per hourper atmosphere, the burning to be carried out at atmospheric pressure or at a pressure of only a few atmospheres, yet with good efiiciency, and practically no smoke or S0 production, i.e., less than Shell Smoke No. 2 and less than p.p.m. of S0 It is another object of this invention to provide a method of burning residual fuels in which the burning is carried out in steps, comprising first burning the fuel at an air-fuel ratio less than that theoretically required for burning under conditions corresponding to stoichiometric conditions and under conditions which will produce a combustible gas, cooling the products of combustion from the burning of the fuel, and then mixing the cooled gas with further air in an amount just suificient to make the air-fuel ratio for the total amount of fuel equal to the ratio theoretically required for burning corresponding to burning at stoichiometric conditions, whereby products of combustion corresponding to burning at stoichiometric proportions of air and fuel are produced which are substantially completely free of S0 It is a further object of the present invention to provide a method of burning residual fuels in which the burning is carried out in steps, comprising first burning a part of the fuel at an air-fuel ratio less than that theo 3,228,451 Patented Jan. ll, 1966 retically required for burning under conditions corresponding to stoichiometric conditions and under conditions which will produce a combustible gas, cooling the products of combustion from the burning of this part of the fuel, and then mixing the cooled gas with the remainder of the fuel which is being burned at an air-fuel ratio greater than that theoretically required for burning under conditions corresponding to st-oichiometric proportions by an amount just sufficient to make the air-fuel ratio for the total amount of fuel equal to the ratio theoretically required for burning corresponding to stoichiometric conditions, whereby products of combustion corresponding to burning at stoichiometric proportions of air and fuel are produced which are substantially completely free of S0 It is a still further object of the present invention to provide a method of carrying out this type of burning in a heat exchange apparatus in which the cooling of the products of combustion of the burning of the fuel is utilized to contribute to the overall heat exchange in the heat exchange apparatus.

Other and further objects of the present invention will become apparent from the following specification ad claims, taken together with the accompanying drawings, in which:

FIG. 1 is a graph showing a theoretical curve for burning of a given residual fuel in which the theoretical instantaneous burning temperature has been plotted against the air-fuel ratios;

FIG. 2 is a diagrammatic sectional view of a heat exchange apparatus particularly suitable for carrying out the method according to the invention; and

FIGS. 3-5 are views similar to that of FIG. 2 of modified forms of heat exchange apparatus particularly suited for carrying out the method according to the invention.

Referring to FIG. 1, the temperatures along the curve shown are the theoretical temperatures at which instantaneous burning of the fuel takes place at the air-fuel ratios shown. It will be seen that the temperature rises as the air-fuel ratio falls toward the ratio of 14, i.e. 14 weight units of air to one weight unit of fuel, at which ratio the proportions correspond to those theoretically necessary for stoichiometric burning. It should be understood that a practical burning curve is about 300 C. lower than this theoretical curve.

At the stoichiometric ratios, the instantaneous burning temperature is near its highest, and thereafter as the ratio decreases further to proportions in which there is excess fuel, the temperature rises only slightly and then falls off.

It Well known that where there is an excess of air sufiicient to insure complete combustion and consequently an excess of oxygen in the burning zone, there is ample opportunity for the S0 produced in the burning to oxidize to S0 This takes place most readily at the instantaneous burning temperatures for the air-fuel ratios close to the stoichiometric ratio. It is now believed that this occurs because at these ratios the temperature of the reaction is sufliciently high to make the oxygen atomic, so that it readily oxidizes the S0 The idea on which the present invention is based is to carry out the combustion reaction so that the temperatures at which burning of the particles of fuel takes place are always well below the temperatures near the peak of the burning temperature curve, so that the S0 is never given a chance to be oxidized, and at the same time carrying out combustion so as to keep the oxygen available for oxidation of the S0 to a minimum. This latter condition is necessary because the flame is not homogeneous and temperatures at localized points may reach temperatures of burning at stoichiometric proportions.

It is well known that temperatures can be reduced either by adding excess air or excess fuel. However, each of these methods by itself has a serious disadvantage, the addition of excess air resulting in poor fuel economy and providing the excess oxygen for the oxidation of the S and the addition of excess fuel causing incomplete combustion and the production of smoke. The problem therefore becomes how to reduce the flame temperature and at the same time reduce the oxygen available for oxidation of S0 while keeping the combustion economical and clean.

The method of the present invention utilizes both of these two methods of controlling the temperature, with the addition of a further cooling step. The method of the invention comprises burning the total fuel to be burned in two separate steps. In the first step, the fuel is burned at an air-fuel ratio less than that for stoichiometric burning, i.e. with an excess of fuel, down to a lower limit of a ratio which is the stoichiometric ratio, the ratio preferably being from /3 to so that for this portion of the burning, the burning will take place at a temperature well below the high temperatures produced when burning is at stoichiometric portions, for example at /3 the stoichiometric ratio at point I on the graph of FIG. 1. Thereafter, the products of this combustion are cooled at least so as to remove a substantial part of the heat therein, as represented by the vertical line, to a. temperature at point II on the graph of FIG. 1. Then air is introduced into the products of combustion of the burning at the low air-fuel ratio in an amount such that the ratio of total air to the fuel is no greater than about in excess of the stoichiometric ratio. This completes the combustion of the fuel, and the temperature of this burning to completion will also be well below the temperature for burning at the stoichiometric ratio, so that the overall temperature of the burning will be below that of the gases produced by burning at a higher air-fuel ratio, while it is above that of the initial burning at the air-fuel ratio less than stoichiometric, the latter products being heated approximately along the dotted line of FIG. 1 from the point II to the vicinity of point III.

Alternatively the method of the invention comprises burning the total fuel to be burned in two separate parts in two separate steps. In the first step, part of the fuel is burned at an air-fuel ratio less than that for stoichiometric burning, i.e. with an excess of fuel, down to a lower limit of a ratio which is the stoichiometric ratio, so that for this portion of the burning, the burning will take place at a temperature well below the high temperatures produced when burning is at stoichiometric proportions, for example at point I on the graph of FIG. 1. Thereafter, the products of the combustion are cooled to remove a substantial amount of heat therefrom, a-s represented by the vertical line, to a temperature at point II, and thereafter they have the remainder of the fuel burned therein at an air-fuel ratio greater than that for stoichiometric burning, i.e. with an excess of air. The temperature of this latter burning will also be below the temperature for burning at the stoichiometric ratio, and the overall temperature of the burning will be below that of the gases produced by burning at the higher ratio but above that of the products gasified at the lower ratio, the latter products being heated approximately along the dotted line of FIG. 1 from the point II to the point III. Moreover, the amount of excess air used in the burning at the higher air-fuel ratio is such that the ratio of the total air to the total fuel is no greater than about 5% in excess of the stoichiometric ratio.

It will thus be seen that the fuel is completely burned, so that there is no smoke produced, but at the same time, the temperature of the burning is kept out of the area of the curve which represents high combustion temperatures by removing a substantial amount of heat during the burning, so that there is no opportunity for the production of atomic oxygen, and therefore no opportunity for the of the combustible gas.

Example I For a sulphur containing residual fuel which requires 14 lbs. of air to burn 1 lb. of fuel, lbs. of fuel are to be burned per unit time. In a normal burner burning with 15% excess air, the amount of air used would be:

1400 lbs.+.15 1400=1610 lbs. air 1610 lbs. air-4400 lbs. air=210 lbs. excess air The burning temperature when burning at this air-fuel ratio of 16.1/1 is about 1950 C. (point IV on the graph of FIG. 1).

In carrying out the method according to the invention, it is required that excess air be no greater than 5%, which represents 1% excess oxygen. Accordingly, the total amount of air for burning 100 lbs. of fuel with 5% excess air is:

1400+.05 X 1400: 1470 lbs. air

The difference between the amount of air which should be used and the amount of air which is used due to burning at 15 excess air is:

1610-- 1470': lbs. air

In order to use up this air, sufficient fuel is introduced into a gasifier to produce combustible gas in an amount just sulficient to utilize the 140 lbs. of air to complete the combustion of the combustible gas. If the gasifier is operated at an air-fuel ratio /2 that which is equivalent to stoichiometric burning, or an air fuel ratio of 7, one-half the fuel will be excess, so that the amount of fuel necessary to use 140 lbs. of excess air at this ratio is:

Since this 10 lbs. of fuel represents /2 of the fuel gasified, it, together with the remaining /2 equal the total fuel supplied to the gasifier. Thus: 10= /2 T, T -20 =10 lbs. fuel for burning excess air As seen in FIG. 1, this mixture will burn at 1700 C.

(point I on the graph of FIG. 1). The products of this combustion, which are a combustible gas, are cooled 400 C. to 1300 C. (point II) and then fuel is burned therein at the air-fuel ratio of 16.1 to complete the combustion The overall burning thus takes place at a temperature no higher than 1850 C. (point III on the graph) and has substantially no S0 therein, yet is free of smoke from incompletely burned fuel.

Example II For a sulphur containing residual fuel which requires 14 lbs. of air to burn 1 lb. of fuel, 1000 lbs. of fuel are to be burned in a number of burners on a boiler per unit time. In the boiler in question, the burners burn with 25% excess air, so that the amount of air used would be:

14,000+.25 14,000=17,500 lbs. air 17,500-l4,000=3,500 lbs. excess air The temperature at which the burning takes place at this air-fuel ratio of 17.5/1 is about 1850 C.

The difference between the amount of air which should be used to produce burning at an overall stoichiometric proportions of air and fuel and the air which is used due to burning at 25% excess air is 3,500 lbs. excess air. In

order to use up this air, sufiicient fuel is introduced into =250 lbs. of fuel for burning excess air Since this 250 lbs. of fuel represents /3 of the fuel gasified, it together with the remaining /3 equal the total fuel supplied to the gasifier. Thus As seen from the graph of FIG. 1, this mixture will burn at 1300 C. The products of this combustion, which are a combustible gas, are cooled, for example 400 C. to a temperature of 900 C. and then have fuel burned therein at the air-fuel ratio of 17.5/1 to complete the com bustion of the combustible gas. The overall combustion thus takes place at a temperature lower than the 1850 C. temperature of the combustion products burned at 17.5/ 1, and has substantially no 80;, therein, yet is free of smoke from incompletely burned fuel.

An apparatus which is particularly adapted to carry out the method according to the present invention is a heat exchange apparatus such as a boiler for heating water to steam. Several examples of how such an apparatus can be used to carry out the aspect of the method of the invention illustrated in Examples I and II are set forth below with special reference to FIGURES 2-4.

In the apparatus of FIG. 2, a heat exchanger 20, such as a boiler, has attached thereto a gasifying chamber 21, the gasifying chamber 21 being surrounded by an air supply casing 22. A part of the heavy residual liquid fuel is gasified in the chamber 21, the conditions in the chamber being controlled by controlling the supply of air from the casing 22 such that there is a deficiency of air and combustible gases are produced.

A particularly satisfactory form of gasifying chamber is one having conical front and rear walls which are perforated for the inlet of combustion air under pressure which is supplied to the casing 22 through the inlets 23, while the liquid fuel is introduced through the front wall of the chamber by an atomiser 24, preferably a two-fluid type of atomiser, at the apex of the front conical wall, the atomiser 24 producing a wide angled cone of spray, The mixture of the atomised fuel, air and combustion gases is so effective and under such control within this chamber by reason of the stable acre-dynamic pattern of flow within it that the composition of the products of thermal decomposition or molecular breakdown, which products are in the form of combustible gases, are steady and controlled even when the gasifier is operated is op erated at a very low air to fuel ratio, in which case there is a large excess of fuel.

The combustible gases produced in the gasifier 21 leave the chamber 21 and pass through the effiux duct 25 through the rear Wall of the chamber and enter the lower part of the combustion chamber 26 of the boiler 20. The gases from the gasifier 21 will be cooled when they enter the chamber at this point, for example from a temperature of 1700 C. to 1300 C., as in Example I.

A second liquid fuel atomiser 27 having an air supply casing 28 and an air supply inlet 29 is provided and proiects a spray of the remainder of the heavy residual fuel into the chamber 26 with excess air as, for example, computed in Example I in the same direction as the combustible gases are put into the chamber 26 by the efilux duct 25. The spray from the atomiser 27 will entrain the gases from the efflux duct 25 and excess air will be used to burn the combustible gases as the spray and gases are mixed along the length of the chamber 26, as described in Example I. The exhaust gases, which will be free of S0 and smoke, leave the chamber 26 by the exit flue 30.

In the arrangements shown in FIGURES 3 and 4, the combustible gases from the gasifying chamber are utilized to drive a gas turbine which in turn drives an air compressor which supplies all the air under pressure required for the combustion process. In FIGURE 3, the combustible gases from the gasifying chamber 21a flow forwardly through the axial heat-interchange part 31 of the heat exchanger 20a in which they are cooled, and then rear- Wardly through the second multi-tubular part 32 thereof before entering the casing 33 of a gas turbine 34, and the remainder of the fuel and additional air for complete combustion being in this case provided by its admittance into the rear end 35 of the exchanger through a fuel atomiser 27a and inlet 2% in the casing 36. The exhaust gases from the turbine 34 pass back into the heat exchanger through the heat interchange part 37 thereof and thence pass to the exit flue 30a. FIGURE 3 shows a particular form of assembly of turbine 34-, air compressor 38, and liquid fuel atomiser 39, all co-axial with the gasifying chamber 21a. The gas turbine 34 is of radial flow design, and theair compressor 38 is of the centrifugal vane wheel type and both are mounted on a hollow shaft 40 through which the liquid fuel is supplied and is discharged into the chamber 21a by the rotary cup-ended atomiser 39, which spins the liquid fuel and spreads it out into a Wide-angled cone corresponding approximately with the conicity of the rear wall of chamber 210:. Air is drawn by the compressor 38 into a casing 41 surrounding the turbine-compressor assembly, and is driven through inlets 42 into the casing 43 around the gasifying chamber 21a, some of it then entering the chamber 21:: in converging jets through the holes in the chamber walls, and the remainder entering the chamber through the annular gap around the spinning cup atomiser 39.

The arrangement according to FIGURE 4 is similar to that of FIGURE 3 as regards the use of a combined turbine 34b, air compressor 38b, and liquid fuel atomiser 39b, but differs therefrom in that the combustible gases passing out the gasifying chamber 21b flow directly through the bent duct 31b and are cooled by heat interchange within the heat exchanger 2%, and then flow to the gas turbine 34b, and thence through the turbine exhaust duct 44 to the combustion chamber 35b of the heat exchanger to which the remainder of the fuel and the requisite additional air (oxygen) for complete combustion at stoichiometric proportions within the chamber 35b is supplied by a duct 45 from the air-compressor 38b and fuel atomiser 27b as in FIG. 2.

It is not essential, as indicated above, that the fuel be burned in two parts. What must be done is to take some steps to remove heat from the process at some intermediate point so that the temperatures do not rise to the point which is favorable for the formation of S0 To this end, the combustion may be carried out by first burning the entire amount of fuel at an air-fuel ratio lower than that for stoichiometric burning, and then cooling the products of this combustion, and thereafter adding sufiicient air to just complete the combustion. An example of this manner of carrying out the invention will now be given.

EXAMPLE III For a sulphur containing residual fuel which requires 14 lbs. of air to burn 1 lb. of fuel, lbs. of fuel are to be burned per hour.

The entire amount of fuel is burned in a first combustion chamber at an air-fuel ratio which is half the air-fuel ratio which corresponds to stoichiometric burning, i.e. at an air-fuel ratio of 7, which requires 700 lbs. of air per hour, and at two atmospheres of pressure.

7 This burning will take place at 1800 C., as seen from FIG. 1.

The products of this combustion are cooled to 800 C., for example by passing them through a heat exchanger, and they then have added to them 700 lbs. of air to bring the total amount of air up to an air-fuel ratio of 14, the ratio corresponding to stoichiometric burning. This can be done by passing the gases through a gas turbine where the temperature is further lowered due to the removal of energy necessary for the running of the turbine, and in which turbine the additional air is added.

The step of cooling the gases from the initial combustion to 800 C. removes sufiicient heat, almost 25% of the total heat which can be delivered by the combustion of the fuel, so that addition of the air necessary just to complete combustion does not release sufficient heat to raise the temperature any higher than the temperature at which the incomplete burning of the fuel took place, so that temperature conditions favorable to the formation of S are never reached. In practice, the temperature at which combustion would be completed in the second step would not be above about 1700 C. All of the combustion therefore takes place well below the peak of the curve of FIG. 1.

An example of an apparatus in which the method of the invention can be carried out in the manner described in Example III is shown in FIG. 5. The apparatus of FIG. is similar to that of FIGS. 3 and 4 as regards the use of the combined turbine 34c, air compressor 38c, and liquid fuel atomiser 39c. As in the embodiment of FIG. 4, the combustible gases passing out of combustion chamber 210, in which all of the fuel is partially burned, fiow directly through the bent duct 31c and are cooled by heat interchange within the heat exchanger 20c, and then flow to the gas turbine 34c and thence through the twin turbine exhaust ducts 440 to the combustion chamber 350 of the heat exchanger. The additional air is supplied to the turbine exhaust duct 44c and through duct 46 and holes 46a, the air being supp-lied to the duct 46 and the casing around the turbine 34b from the air compressor 380. In addition to the casing 410 with the inlets 42c, the compressor of the embodiment of FIG. 5 has a liquid fuel feed pump 47 connected to the fuel feed pipe 400, the pump 47 being :driven by gearing from the starting motor 48 to start U116 pumping of fuel tothe apparatus.

It is thought that the invention and its advantages will be understood from the foregoing description and it is apparent that various changes may be made in the form, construction and arrangement of the parts without departing from the spirit and scope of the invention or sacrificing its material advantages, the forms hereinbefore described and illustrated in the drawings being merely preferred embodiments thereof.

We claim:

1. A method of burning residual fuel oils containing sulphur for eliminating the production of S0 comprising the steps of first burning at least a part of the fuel at an air fuel ratio less than that theoretically required for burning under conditions corresponding to stoichiometric conditions, said burning being at a practical burning temperature which is less than the temperature at which S0 is formed from S0 and which burning is under conditions which will produce a combustible gas, removing a quantity of heat from the combustible gas which is produced by such burning sufiicient to lower the temperature of the combustible gas to a temperature such that when further air is added to the combustible gas to complete the combustion of the fuel, the products of combustion will be heated only to a temperature which is less than the temperature at which S0 is formed from S0 and which temperature is substantially lower than the practical temperature of burning of the fuel at an air fuel ratio corresponding to stoichiometric burning, and then completing the combustion of the fuel by introducing into the said combustible gas with the heat removed therefrom at least an amount of further air just sufiicient to make the air-fuel ratio for the said part of the fuel substantially equal to the ratio theoretically required for burning at stoichiometric conditions, whereby products of combustion corresponding to burning at stoichiometric proportions of air and fuel are produced at burning temperatures less than the temperature at which S0 is formed from S0 and which products of combustion are substantially free of S0 2. A method of burning residual fuel oils containing sulphur for eliminating the production of S0 comprising the steps of first burning a part of the fuel at an air fuel ratio less than that theoretically required for burning under conditions corresponding to stoichiometric conditions and under conditions which will produce a combustible gas, removing heat from the combustible gas which is produced by such burning and then burning in an amount suflicient to lower the temperature of the combustible gas to a temperature such that when further air and the remainder of the fuel are added to the combustible gas to complete the combustion of the fuel, the products of combustion will be heated only to a temperature substantially lower than the temperature of instantaneous burning of the fuel at an air fuel ratio corresponding to stoichiometric burning the remainder of the fuel in said gas with the heat removed therefrom at an air-fuel ratio greater than that theoretically required for burning under conditions corresponding to stoichiometric conditions, said ratio being greater by an amount just sufiicient to make the airfuel ratio for the total amount of fuel substantially equal to the ratio corresponding to stoichiometric conditions, whereby products of combustion corresponding to burning at stoichiometric proportions of air and fuel are produced which are substantially free of S0 3. A method of burning residual fuel oils as claimed in claim 2 in which the step of first burning a part of the fuel is carried out at an air-fuel ratio of from 73 to the air-fuel ratio corresponding to burning at st-oichiometric conditions, and the step of removing heat from the combustible gas comprises removing sufficient heat to lower the temperature thereof by from 400 C. to 1000 C.

4. A method of burning residual fuel oils as claimed in claim 2 in which the step of first burning a part of the fuel is carried out at an air-fuel ratio of from /3 to the air-fuel ratio corresponding to burning at stoichiometric conditions, the step of removing heat from the combustible gas comprises removing sufiicient heat to lower the temperature thereof by from 400 to 1000 C., and the step of burning the remainder of the fuel is carried out at an air-fuel ratio which is just sufficient to make the airfuel ratio for the total amount of fuel no greater than that which will provide an excess of 5% air greater than that corresponding to the air necessary for burning at stoichiometric proportions.

5. A method of burning residual fuel oils containing sulphur for eliminating the production of S0 comprising the steps of first burning the fuel at an air-fuel ratio less than that theoretically required for burning under conditions corresponding to stoichiometric conditions, said burning being at a practical burning temperature which is less than the temperature at which S0 is formed from S0 and which burning is under conditions which will produce a combustible gas, removing a quantity of heat from the combustible gas which is produced by said burning tsufiicient to lower the temperature of the combustible gas to a temperature such that when further air is added to the combustible gas to complete the combustion of the fuel, the products of combustion will be heated only to a temperature which is less than the temperature at which S0 is formed from S0 and which temperature is substantially lower than the practical temperature of burning of the fuel at an air fuel ratio corresponding to stoichiometric burning, and then completing the combustion by adding further air to said gas with the heat removed therefrom in an amount just sufiicient to make the air-fuel ratio for the total amount of fuel substantially equal to the ratio theoretically required for burning at stoichiometric conditions, whereby products of combustion corresponding to burning at stoichiometric proportions of air and fuel are produced at burning temperatures less than the temperature at which S0 is formed from S0 and which products of combustion are substantially free of S0 6. A method of burning residual fuel oils as claimed in claim 5 in which the step of first burning the fuel is carried out at an air-fuel ratio of from /2 to /3 the air-fuel ratio corresponding to burning at stoichiometric conditions, and the steps of removing heat from the combustible gas comprises removing suflicient heat to lower the temperature thereof by from 400 C. to 1000" C.

7. A method of burning residual fuel oils as claimed in claim 5 in which the step of first burning the fuel is carried out at an air-fuel ratio of /2 that corresponding to burning at stoichiometric conditions, and the step of removing heat from the combustible gas comprises removing sufficient heat to lower the temperature thereof by from L900 C.

References Cited by the Examiner UNITED STATES PATENTS 2,927,632 3/1960 Fraser 1584 JAMES W. WESTHAVER, Primary Examiner. 

1. A METHOD OF BURNING RESIDUAL FUEL OILS CONTAINING SULPHUR FOR ELIMINATING THE PRODUCTION OF SO3, COMPRISING THE STEPS OF FIRST BURING AT LEAST A PART OF THE FUEL AT AN AIR FUEL RATIO LESS THAN THAT THEORETICALLY REQUIRED FRO BURNING UNDER CONDITIONS CORRESPONDING TO STOICHIOMETRIC CONDITIONS, SAID BURNIN BEING AT A PRACTICAL BURNING TEMPERATURE WHICH IS LESS THAN THE TEMPERATURE AT WHICH SO3 IS FORMED FROM SO2, AND WHICH BURNING IS UNDER CONDITIONS WHICH WILL PRODUCE A COMBUSTIBLE GAS, REMOVING A QUANTITY OF HEAT FROM THE COMBUSTIBLE GAS WHICH IS PRODUCED BY SUCH BURNING SUFFICIENT TO LOWER THE TEMPERATURE OF THE COMBUSTIBLE GAS TO A TEMPERATURE SUCH THAT WHEN FURTHER AIR IS ADDED TO THE COMBUSTIBLE GAS TO COMPLETE THE COMBUSTION OF THE FUEL, THE PRODUCTS OF COMBUSTION WILL BE HEATED ONLY TO A TEMPERATURE WHICH IS LESS THAN THE TEMPERATRUE AT WHICH SO3 IS FORMED FROM SO2, AND WHICH TEMPERATURE IS SUBSTANTIALLY LOWER THAN THE PRACTICAL TEMPERATURE OF BURNING OF THE FUEL AT AN AIR FUEL RATIO 